Some submersed fluid pumps have pumping action that is based on linear reciprocal motion. For example, diaphragm pumps may use a reciprocating hydraulic rod to displace fluid, which alternately inflates and deflates a diaphragm within the fixed volume of a pump casing. One-way inlet and discharge (outlet) valves take advantage of the changes in volume between the fixed casing and the expanding and contracting diaphragm to pump well fluid in desired flow paths. As the diaphragm deflates within the pumping chamber, an inlet check valve allows well fluid to enter the casing. Then, as the diaphragm inflates, the pressure is raised within the casing until the discharge check valve opens to allow the pumped well fluid out, for example, into an underground pipe conveying the well fluid to the surface. When compressible fluids (gases and gases-liquid mixtures) enter the pumping chamber, the reciprocating motion may be wasted compressing this kind of well fluid, and the compression obtained is not sufficient to open the discharge check valve and pump out the well fluid. This condition is referred to as “gas interference” or “gas lock.”